This page converts atmospheres into inches of water using the factor 406.7824781, and shows the reverse figure alongside it. Both units measure pressure, which is why one number does the whole job.
The low-pressure unit for duct static pressure and gas appliances in the US. Standard atmospheric pressure by definition — 101,325 pascals exactly.
Gauge versus absolute. A tyre gauge reading 32 psi means 32 psi above atmospheric, which is about 46.7 psi absolute. Converting the number is straightforward; remembering which zero it was measured from is where the mistakes happen. N/mm² and MPa are the same size, so a drawing quoting either is quoting the same stress. Below the calculator there is a step table for the values people look up most, and a second table putting real pressure figures into inches of water so the number you get has something to be compared against.
The formula
atmospheres- The amount you are converting, in atmospheres
406.782478063- The exact number of inches of water in one atmosphere
inches of water- The converted result
How it works, step by step
- Enter the value in atmospheres.
- It is multiplied by 406.782478063, the exact factor between the two units.
- The answer is shown in inches of water, with the reverse conversion beneath it.
- Check the reference table if you want a figure of comparable size.
Worked examples
Sea-level air: 1 atm in inches of water
Take sea-level air, roughly 1 atm. The conversion is a single multiplication — 1 × 406.782 — giving 406.782 inH₂O. Because the relationship is proportional, doubling the input doubles the answer to 813.565 inH₂O.
A bicycle tyre: 6.12 atm in inches of water
At 6.12 atm (a bicycle tyre), the same quantity in inches of water is 6.12 × 406.782 = 2489.51 inH₂O. One atmosphere is 406.782 inches of water, so the numbers grow as expected.
How to read your score
Frequently asked questions
How many inches of water are in one atmosphere?
Exactly 406.782478063. Every figure on this page is that one number multiplied by your input, so the only rounding is at the display step.
What is the formula to convert atmospheres to inches of water?
inches of water = atmospheres × 406.782478063. Reversed, atmospheres = inches of water ÷ 406.782478063, which is the same as multiplying by 0.00245831630891.
Which is the bigger unit, the atmosphere or the inch of water?
The atmosphere. One atmosphere equals 406.782 inches of water.
What does this quantity actually measure?
Both units measure pressure. That is why a single factor is enough: there is no temperature offset and no material property involved, unlike conversions that cross between different quantities.
What is a atmosphere?
Standard atmospheric pressure by definition — 101,325 pascals exactly.
What is a inch of water?
The low-pressure unit for duct static pressure and gas appliances in the US.
Why does my answer differ from another converter?
Almost always because the other tool rounded the factor before multiplying. This page keeps full double precision through the calculation and rounds only for display, and it prints the factor it used so you can see where any difference came from.
Is there a quick way to do atm to inH₂O in my head?
Plenty of people do this in their head: multiply by 400 instead of using the exact factor 406.7824781. At 100 that shortcut gives 40,000 inches of water, off by 678.2 inches of water or about 1.67%. Fine for a rough estimate, not fine for anything you are cutting to size or dosing.
Atmospheres to inches of water reference tables
| Atmospheres | Inches Of Water |
|---|---|
| 0.5 atm | 203.391 inH₂O |
| 1 atm | 406.782 inH₂O |
| 2 atm | 813.565 inH₂O |
| 3 atm | 1220.35 inH₂O |
| 5 atm | 2033.91 inH₂O |
| 10 atm | 4067.82 inH₂O |
| 20 atm | 8135.65 inH₂O |
| 25 atm | 10,169.6 inH₂O |
| 50 atm | 20,339.1 inH₂O |
| 75 atm | 30,508.7 inH₂O |
| 100 atm | 40,678.2 inH₂O |
| 250 atm | 101,696 inH₂O |
| 500 atm | 203,391 inH₂O |
| 1000 atm | 406,782 inH₂O |
| 5000 atm | 2.03391e+06 inH₂O |
Exact factor: 1 atmosphere = 406.782478063 inches of water.
| Quantity | atm | inH₂O |
|---|---|---|
| Sea-level air | 1 atm | 406.8 inH₂O |
| A car tyre | 2.177 atm | 885.8 inH₂O |
| Blood pressure, systolic | 0.1579 atm | 64.23 inH₂O |
| A bicycle tyre | 6.124 atm | 2491 inH₂O |
| A domestic water main | 3.948 atm | 1606 inH₂O |
| A steam boiler | 9.869 atm | 4015 inH₂O |
Published reference values, converted with the same factor this page uses.